Process and arrangement for photothermal spectroscopy

ABSTRACT

A process and arrangements for photothermal spectroscopy (thermal wave analysis) by the single-beam method with double modulation technique. A single-beam method is developed making use of the advantages of double modulation technique in detecting the photothermally generated difference frequency without requiring partial beams and while achieving extensive absence of intermodulation, the intensity of the laser beam is modulated before striking the object in such a way that the modulation spectrum substantially contains a carrier frequency (f 1 ) and two sideband frequencies (f 1  ±F 2 ), wherein f 2  is the base clock frequency of the modulation, a regulating detector and a control loop intervening in the modulation process suppress that component of the base clock frequency (f 2 ) in the same phase with the mixed frequency of the carrier frequency and sideband frequencies. After interaction with the object the optical response of the object is measured by means of a measurement detector and frequency-selective and phase-selective device as the amplitude of that component of the base clock frequency (f 2 ) which, as the photothermal mixed product, has the same phase as the mixed frequency of the carrier frequency (f 1 ) and sideband frequency (f 1  ±f 2 ). Use for nondestructive and noncontact analysis of the material parameters of areas of solid bodies close to the surface is described.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention is directed to a process and arrangements for photothermalspectroscopy (thermal wave analysis) based on the single-beam methodwith double modulation technique. It is applied for measuringgeometrical, thermal, electronic and elastomechanical materialparameters of surface coats or layers by evaluating the photothermalresponse signals from areas of solid bodies close to the surface. Thenoncontact and nondestructive process according to the invention isapplied chiefly as a test method for quality control in coatingtechnology.

b) Background Art

Methods of photothermal spectroscopy for noncontact, nondestructivedetection of parameters of thin layers are known. The physicalprinciples and fundamentals are compiled and described e.g. in"Photoacoustic and Thermal Wave Phenomena in Semiconductors" A MANDELIS(Ed.), North Holland, N.Y. 1987. In a known method according toROSENCWAIG, a periodically intensity-modulated pump laser produces aphotothermal response in the layer which in turn locally modulates therefractive index so that the modulated optical reflection (MOR) can bemeasured with a so-called probe laser beam (U.S. Pat. No. 4,579,463).This method accordingly requires two lasers with different wavelengthsand a precise alignment of the two beams relative to one another on thesample. In addition to the optical precision of the beam adjustmentmentioned above, the solutions suggested in U.S. Pat. No. 4,634,290,U.S. Pat. No. 4,636,088, and EP 0 291 276 involve considerable expensefor optical elements for adapting the thickness of the two beam bundles.Further, the inherent noise of the probe laser represents a limitingfactor for the resolution capability requiring the use of lasers withextensive noise stabilization.

Single-beam methods are also known for measuring the MOR (CHEN et al.,Appl. Phys. Lett. 50 (1984) 1349; A. LORINCZ, L. ANDOR, Appl. Phys. B 47(1988) 35; M. WAGNER, H. D. GEILER, Meas. Sci. Technol. 2 (1991) 1088).To separate the MOR from the reflected modulated pump intensity, LORINCZmakes use of the fact that the photothermal response causes harmonicwaves (second harmonic) in the reflected component which can be detectedby lock-in detection. However, the requisite absence of harmonic wavesof 10⁻⁷ cannot be achieved because of the finite nonlinearities of themodulator. This is also true with respect to compensating for noise. Inthe method used by WAGNER, this disadvantage is overcome by applyingdouble modulation technique in which two modulation frequencies areimpressed on the pump beam and sum or difference frequencies produced bythe photothermal refractive index modulation in the sample are detected.However, it is only possible to generate the double-modulated beamwithout intermodulation by combining two separately modulated partialbeams. Beyond the extra expenditure on optics for aligning the partialbeams, this requires the use of two separate optical modulation systems.In particular, the return of the modulated partial beam components intothe laser and accordingly internal modulation of the latter must beprevented. This requires expensive optical insulators.

OBJECT AND SUMMARY OF THE INVENTION

The object of the present invention is to develop a single-beam methodwhich makes use of the advantages of double modulation technique withoutrequiring partial beams and which achieves a high degree of freedom fromintermodulation.

The basic idea of the invention consists in that the stimulation of theobject and the detection of the transmission or reflection modulated bythe object is effected with an individual optical beam directed on theobject, the intensity of the beam being substantially modulated withthree frequencies, one of which is the arithmetical mean of the othertwo frequencies. Thus, the modulation spectrum substantially includes acarrier frequency and its first two sideband frequencies at a distancefrom a base frequency. The thermal wave reaction induced in the objectby means of an optical beam modulated with this frequency spectrum takesplace in the radiation leaving the object (directed, transmitted orreflected component, scatter radiation, heat radiation) in the form ofmixed products of the three frequencies from which the base frequency isdetected by means of a frequency-selective and phase-selective measuringdevice and serves as a measurement of the linear response of the object.

The base frequency is advisedly as small as possible so that the objectis effectively stimulated with the carrier frequency f₁, i.e. thephotothermal dispersion in the vicinity of the carrier frequency isnegligible and the frequency to be detected is in the lower frequencyrange which can be managed easily in terms of technique.

A familiar general problem in intensity modulation of optical radiationis the nonlinear characteristic of the modulator. When using anindividual modulating optical element for generating the modulationspectrum according to the invention, these nonlinearities result in theinclusion of frequency mixed products in the beam directed on the objectwhich complicates and distorts the detection of the thermal waveresponse. For the purposes of the modulation spectrum according to theinvention, a frequency corresponding to the base frequency occurs assuch a mixed product caused by the nonlinearities of the modulationcharacteristic line.

The essence of the invention consists in detecting this disturbingsignal of the base frequency of the beam directed on the object by meansof a regulating detector and removing it via a control loop bymanipulating the modulation spectrum by means of a suitable, purposefulchange in the electrical signal effecting the modulation. This meansthat a component of the base frequency is additionally supplied to themodulation process so as to compensate for the unwanted component ofthis frequency generated by the nonlinearities. This compensation of themodulator nonlinearities is governed by the modulation principle forgenerating a modulation spectrum.

The object is to produce a time function for the modulation of the laserbeam which permits a highly accurate manipulation of the frequencyspectrum with a comparatively small expenditure on technical means and,at the same time, offers a high mixing efficiency for detecting theobject response.

A modulation signal whose base period T is formed from a base frequencyhas proven particularly advantageous in this respect. This base periodis composed of a (proportional or compact) first period within whichthere occurs a square-wave modulation of the intensity of the laser beamwith a shift of 100% with the carrier frequency, wherein a quantity ofperiods of duration of the reciprocal carrier frequency corresponding toa whole number is contained in the first partial period, and by a secondpartial period (occupying the remaining portion of the base time period)in which the intensity of the laser beam is constant.

For an intensity time curve of this type, the frequency component whichis in phase with the base frequency is determined in the frequencyspectrum substantially by the value of the intensity in the second timesegment which is to be adjusted in such a way that the component iszero. For large frequency ratios of the carrier frequency to the basefrequency the intensity value to be adjusted corresponds approximatelyto the arithmetical mean of the intensity formed over the first partialperiod. Under this condition, the modulation spectrum substantiallycontains the frequency with two sideband frequencies at a distance fromthe base frequency of the carrier frequency.

The nonlinear response of the object causes in the radiation leaving theobject a change of the original ratio of the mean values of theintensities in the two partial periods of the base period. This changeis particularly pronounced when the partial periods are of equalduration. It causes a frequency component to be formed which is in phasewith the base frequency. In the frequency diagram, this component isbrought about on the nonlinear response characteristic line of theobject by mixing the carrier frequency with its first sidebandfrequencies.

When used for detecting the thermal wave signals, which are usually verysmall in practice, and accordingly for detecting components of the basefrequency it is impossible to effect a static adjustment of the requiredconstant value of the intensity in the second partial period of the baseperiod with the required accuracy. In practice, this means that afrequency component in phase with the base frequency will alwaysinevitably occur. The frequency component in the modulation spectrum ofthe beam directed on the object, which component is in phase with thebase frequency and thus constitutes interference, is positivelyeliminated, according to claim 1, by means of a control loop whichadjusts the required intensity ratios in the two partial periods of thebase period, e.g., by means of a corresponding change, in the constantvalue of the intensity within the second partial period or by changingthe first pulse-duty factor within the partial period.

For this purpose, in the regulating process, a portion of the modulatedbeam is optically decoupled and the component of the unwanted frequencycomponent in phase with the base frequency f₂ is determined by means ofa photoreceiver (regulating detector) and an additionalfrequency-selective and phase-selective device. This measurement valueis now supplied to the modulator group as an interference variable andcauses an exact modulation signal to be generated.

The regulating process described herein has another advantageous result.It not only reduces the unwanted frequency component of the basefrequency produced during the intensity modulation by nonlinearities inthe modulation process, but at the same time also reduces thosecomponents of frequency in phase with the base frequency which originatefrom the noise of the laser beam intensity and raise the sensitivitylimit of the process, since they are superimposed on the measurementsignal.

Accordingly, compared to known methods for thermal wave analysis, therealization of the process according to the invention on the one handallows a considerable reduction in cost on optics (only an individuallaser source and guidance of an individual optical beam) and on theother hand enables an increase in sensitivity or the possibility ofusing comparatively noisy laser sources.

The range of uses of the process according to the invention can also beextended to photothermal wave reactions of the object which essentiallydo not cause any modulation of the transmission or reflection capabilityor occurring secondary radiation (scattered light, heat radiation), butrather lead to modulation of the position and/or shape of the beam. Thisis done by inserting diaphragms at a suitable location in the path ofthe beam influenced by the object for the purpose of trimming the beamdepending on position and shape.

The application of the process according to the invention in differentarrangements which fully deliver the promised effects may bedistinguished substantially according to the techniques by which theintensity modulation and manipulation of the modulation spectrum arerealized.

Thus, the modulation signal of the type according to the invention isgenerated either in that the light from a source is modulated or in thatthe intensity modulation is produced by suitable modulation of the lightof two beams and the beam directed on the object is generated bycombining the two partial beams.

The modulation spectrum is influenced either in that a correction signalis additionally supplied to the element used for intensity modulation orin that an additional optical element supplied with a correction signalis inserted in the beam path.

For a better understanding of the present invention, reference is madeto the following description and accompanying drawings while the scopeof the invention will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the basic construction, according to the invention, of anarrangement for thermal wave analysis in the transmission variants usinga laser source 1 for generating beams and an acousto-optical modulator 2for intensity modulation;

FIG. 2 shows the basic construction, according to the invention, of anarrangement for thermal wave analysis in the reflection variants using alaser diode 11 and the special square-wave modulation of the intensityfor generating the modulated laser beam;

FIG. 3 shows the control signal supplied to the laser diode 11 in FIG. 2as a function of time;

FIG. 4 shows a special embodiment form of the portion for generating themodulated laser beam by means of two acousto-optical modulators 2 and12, which are arranged one after the other in the beam path;

FIG. 5 shows a special embodiment form of the portion for generating themodulated laser beam, by means of an integrated-optical arrangementusing an optical frequency modulator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the arrangement according to FIG. 1, the laser beam is generated byan optional laser source 1 and passes through an acousto-opticalmodulator 2 for intensity modulation, the latter being suppliedsubstantially with the modulation frequencies f₁ and f₁ ±f₂ from anelectronic modulator group 8. The modulated beam, which is focussed withthe focusing lens 4, impinges on the object 5 whose response is to bedetermined.

Before the laser beam strikes the object 5, a component of this laserbeam is decoupled to a photoreceiver serving as regulating detector 6 bymeans of a beam splitter 3. The component of the unwanted component ofthe base frequency f₂ is measured in the modulated optical beam by meansof a lock-in amplifier 7, which acts as a frequency selective and phaseselective device and whose frequency and phase are tuned to the basefrequency f₂ likewise supplied by the modulator group 8. The measurementvalue is fed to the group 8 as an interference variable and causes to begenerated therein a modulation component with base frequency f₂ which islikewise supplied to the acousto-optical modulator 2.

The acousto-optical modulator 2, regulating detector 6, lock-inamplifier 7, and electronic modulator group 8 form a control loop whichcauses a component of frequency f₂ to be supplied to the acousto-opticalmodulator which component is defined in such a way that the component ofthe base frequency f₂ produced from the carrier and sideband frequenciesf₁ and f₁ ±f₂ by its modulation nonlinearities is precisely compensatedfor. In this way, the modulation spectrum of the beam directed on theobject 5 does not contain the base frequency f₂.

The component of base frequency f₂ produced in the object 5 as a resultof the photothermal modulation of the refractive index by mixing thecarrier frequency f₁ with its first sideband frequencies f₁ ±f₂ isdetermined by a photoreceiver serving as measurement detector and by alock-in detector 10 which is likewise tuned in frequency and phase tothe base frequency f₂ supplied by the modulator group 8.

In the arrangement according to FIG. 2, a linearly polarized laser beamis generated by a laser diode 11 and directed on the object 5 throughthe focusing lens 4. Before the laser beam strikes the object 5, aportion of this beam is decoupled by means of a beam splitter 3 to aphotoreceiver serving as a regulating detector 6 as set forth in thefirst example.

For the purpose of intensity modulation, the signal shown in FIG. 3 issupplied to the laser diode by an electronic modulator group 8. The timesegments T₁ and T₂ are of identical length for this signal. Theelectronic modulator group 12 has a control input for adjusting thestrength of the signal generated in the time segment T₂.

In order to suppress the unwanted interference frequency in phase withthe base frequency f₂ in the beam directed on the object 5, theregulating detector 6 and the lock-in amplifier 7 which is tuned to thebase clock frequency F₂ =1/T₂ with respect to frequency and phaseposition (see FIG. 3) generate an interference signal which changes thestrength of the signal generated in time segment T₂ via the controlinput of the modulator group 8, so that the component of base frequencyf₂ in the modulated beam is changed. This control loop removes theunwanted component of base frequency f₂ from the modulation spectrum.Before impinging on the object 5, the modulated beam which is generatedin this way passes through a polarizing splitter 13 and a λ/4-wave plate14. The beam component reflected by the object 5 is guided to thephotoreceiver 9 serving as measurement detector 9 by means of thepolarizing splitter 13.

The nonlinear optical response of the object 5 is then determined, asset forth in the first example, by means of the lock-in detector 10which is tuned in frequency and phase to the base frequency f₂ suppliedby the modulator group 8.

FIG. 4 shows an arrangement for generating the modulated beam in whichthe unwanted component of frequency f₂ in the modulated beam iscorrected by using two acousto-optical modulators. The laser beam isgenerated by the laser source 1 and first passes through theacousto-optical modulator 2 which is supplied with frequencies f₁ and f₁±f₂ by the modulator group 8 as was described with reference to thefirst embodiment example. This modulator group also provides the basefrequency f₂.

The beam then passes through another acousto-optical modulator 12. Thismodulator is supplied, via another electronic modulator group 16, with amodulation signal of base frequency f₂ which is derived from the signalof base frequency f₂ supplied by the modulator group 8 and whoseamplitude can be changed via a control input.

A portion of the modulated laser beam is decoupled via the beam splitter3 to the regulating detector 6. As was already described in thepreceding examples, an interference signal is formed by means of thelock-in amplifier 7 which is tuned in frequency and phase to the basefrequency f₂ supplied by the modulator group 8. Via the control input ofthe modulator group 16, this interference signal controls the amplitudeof the additional modulation of the laser beam with the base frequencyf₂ effected by the acousto-optical modulator 12. In so doing, thecontrol is effected so as to compensate for the unwanted component ofthe base frequency f₂ in the beam.

FIG. 5 shows an integrated-optical realization of the modulationprinciple. The laser beam is generated by the laser source 1 and splitinto two partial beams by means of a 3 dB expander 17. One of the twopartial beams passes through a controllable optical phase shifter 18,and the other partial beam passes through an optical frequency modulator19. The two partial beams are then reunited by another 3dB expander 20.The laser beam is modulated in that the modulation period of durationT=1/f₂ is composed of time segments T₁ and T₂. The optical frequencymodulator 19 is acted upon within T₁ by the electronic modulator group21 with a control signal such that the optical frequency of the partialbeam passing through the optical frequency modulator 19 is offset by thevalue of frequency f₁ from the optical frequency of the original laserbeam. After the two partial beams are united in the expander 20, a laserbeam is formed whose intensity is modulated with carrier frequency f₁.

Within T₂, the electronic modulator group 21 forms a control signal suchthat the frequency displacement of the two partial beams is zero.Accordingly, after the two partial beams are joined in the expander 20,a beam is formed with an intensity which is constant with respect totime and whose level depends on the optical phase displacement of thetwo partial beams. Within time segment T₂, the electronic modulatorgroup 21 controls the optical phase shifter 18 in such a way that adetermined phase displacement is adjusted which leads to the intensityrequired for eliminating the interfering component of base frequency f₂.

As set forth in the preceding examples, this control is carried out inthe form of a control loop by way of detecting the interfering componentof base frequency f₂ in the modulated beam.

While the foregoing description and drawings represent the preferredembodiments of the present invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the true spirit and scope of the presentinvention.

What is claimed is:
 1. A process for photothermal spectroscopy in whichan intensity-modulated laser beam is directed on an object to beexamined and in which, as a result of a reaction of the object to energyapplied by the laser beam after interaction with the object, anadditional modulation of the intensity is determined as an opticalresponse of the object by a measurement detector, comprising the stepsof performing:a) intensity modulation of the laser beam before itimpinges on the object, whose modulation spectrum substantially containsonly a carrier frequency (f₁) and two sideband frequencies (f₁ ±f₂), thesideband frequencies (f₁ ±f₂) resulting as an amplitude modulation of abase frequency (f₂), b) eliminating a component of the base frequency(f₂) having the same phase as a mixed frequency resulting from thecarrier frequency (f₁) and sideband frequencies (f₁ ±f₂) by means offeeding back said detected component as an interference variable to themodulation step, and c) measuring the amplitude of that component of thebase frequency (f₂) which is contained in the beam leaving the object asa photothermal modulation product in phase with the mixed frequency ofthe carrier frequency (f₁) and sideband frequencies (f₁ ±f₂) by the useof a frequency-selective and phase-selective device.
 2. The processaccording to claim 1, wherein the phase relation of frequency f₁ and itssidebands f₁ ±f₂ is periodically reversed by 180° at a frequency whichis less than frequency f₂.
 3. The process according to claim 1, whereinthe component of frequency f₂ which is contained in the modulationspectrum of the intensity of the laser beam directed on the object andin quadrature with the mixed frequency of the carrier frequency andsideband frequencies is also detected by means of anotherfrequency-selective and phase-selective device and is supplied as aninterference signal to another control loop influencing the modulationspectrum for the purpose of adjusting the detected component to thevalue of zero.
 4. The process according to claim 1, wherein the laserbeam is generated by an intensity-modulated laser diode.
 5. The processaccording to claim 1, wherein the laser beam passes one or more opticalmodulators in the beam path between the laser source and the object. 6.The process according to claim 1, wherein the intensity modulation iseffected in that the laser beam is composed of two partial beamssusceptible to interference, the mean values of the optical frequenciesof the two partial beams differing by the value of frequency f₁, and theoptical frequency of one partial beam is frequency-modulated with asignal of the base frequency f₂.
 7. The process according to claim 1,wherein the intensity modulation is effected in that the laser beam iscomposed of two partial beams susceptible to interference, the meanvalues of the optical frequencies of the two partial beams differing bythe value of frequency f₁, and the optical phase position of one partialbeam is phase-modulated with a signal of base frequency f₂.
 8. Theprocess according to claim 1, wherein the intensity modulation of thelaser beam is effected in such a way that a modulation period ofduration T=1/f₂ comprises a time segment of duration T₁, in which theintensity is modulated with a periodic signal of carrier frequency f₁,and a time segment of duration T₂, in which the intensity is constant,and its value is adjusted by said control loop so that the component ofthe frequency spectrum in phase with frequency f₂ has an intensity ofzero.
 9. The process according to claim 8, wherein an integral number ofperiods of duration 1/f₁ is contained within the time segment T₁. 10.The process according to claim 9, wherein the modulation with frequencyf₁ within time segment T₁ is effected in such a way that the start ofthis time segment does not coincide with a zero axis crossing of theoscillation of frequency f₁.
 11. The process according to claim 3,wherein the influencing of the modulation spectrum by changing the timeperiod is effected between the start of time segment T₁ and the zeroaxis crossing of the oscillation of frequency f₁.
 12. The processaccording to claim 8, wherein the time segments T₁ and T₂ are of equalduration.
 13. The process according to claim 8, wherein the influencingof the modulation spectrum by changing the intensity is effected withintime segment T₂.
 14. The process according to claim 8, wherein asquare-wave modulation is effected within the time segment T₁.
 15. Theprocess according to claim 14, wherein the modulation spectrum isinfluenced by changing the pulse-duty factor.
 16. The process accordingto claim 14, wherein the modulation spectrum is influenced by changingthe rise time and fall time of the square-wave pulses.
 17. The processaccording to claim 6, wherein a modulation form is produced in that thelaser beam is composed of two partial beams susceptible to interference,the optical frequency of one partial beam being modulated with a signalof period T in such a way that it differs from the optical frequency ofthe other partial beam in time segment T₁ by the value of frequency f₁.18. The process according to claim 7, wherein a modulation form isproduced in that the laser beam is composed of two partial beamssusceptible to interference, the optical phase position of one partialbeam being modulated with a signal of period T in such a way that thephase difference changes periodically relative to the other partial beamin time segment T₁ with the base frequency f₁ and remains constant intime portion T₂.
 19. The process according to claim 1, wherein aproportion of the light directed on the object is decoupled to areference detector for generating a reference value and the differentialsignal is evaluated by the measurement detector and reference detector.20. The process according to claim 19, wherein a regulating detector isalso used simultaneously as a reference detector.
 21. In an arrangementfor photothermal spectroscopy in which an intensity-modulated laser beamis directed on an object to be examined and in which, as a result of areaction of the object to energy applied by the laser beam afterinteraction with the object, an additional modulation of the intensityis determined as an optical response of the object by a measurementdetector, the improvement comprising:a) means for performing intensitymodulation of the laser beam before it impinges on the object, whosemodulation spectrum substantially contains only a carrier frequency (f₁)and two sideband frequencies (f₁ ±f₂), the sideband frequencies (f₁ ±f₂)resulting as an amplitude modulation of a base frequency (f₂); b) meansfor eliminating a component of the base frequency (f₂) having the samephase as a mixed frequency resulting from the carrier frequency (f₁) andsideband frequencies (f₁ ±f₂) by means of feeding back said detectedcomponent as an interference variable to the modulation means; and c)means for measuring the amplitude of that component of the basefrequency (f₂) which is contained in the beam leaving the object as aphotothermal modulation product by the use of a frequency-selectivedevice.
 22. The arrangement of claim 21, wherein said amplitudemeasuring means measures the photothermal modulation product in phasewith the mixed frequency of the carrier frequency (f₁) and sidebandfrequencies (f₁ ±f₂) wherein the device is also phase selective.
 23. Thearrangement of claim 21, wherein said means for eliminating a componentof the base frequency includes a semireflecting mirror arranged afterthe modulation means for decoupling a partial beam of the modulatedlaser beam to a regulating detector, said regulating detector beingconnected with an electronic modulator group at an output side via alock-in amplifier acting as a frequency-selective and phase-selectivedevice, said modulator group being coupled to a control input of saidmodulator means.
 24. The arrangement of claim 22, wherein said means formeasuring includes a lock-in detector acting as a frequency-selectiveand phase-selective device and being arranged in the beam path afterinteraction of the laser beam with the object, said lock-in detector isadapted to be timed to the base frequency.
 25. The arrangement of claim21, wherein said laser source and modulator means are combined in anintensity-modulated laser diode.
 26. In an arrangement for photothermalspectroscopy in which an intensity-modulated laser beam is directed onan object to be examined and in which, as a result of a reaction of theobject to energy applied by the incident laser beam, the objectimpresses an additional modulation of intensity on the laser beam in thepath of the beam after interaction with the object, a measurementdetector being arranged for detecting this additional modulation as anoptical response of the object, the improvement comprising:a lasersource and a modulator which are provided for generating theintensity-modulated laser beam, said modulator having a modulationspectrum substantially containing only three frequencies in the form ofthe carrier frequency (f₁) with the first sidebands (f₁ ±f₂), whichthree frequencies result from a carrier frequency (f₁) and a basefrequency (f₂); a semireflecting mirror which is arranged after themodulator for decoupling a partial beam of the modulated laser beam to aregulating detector, said regulating detector being connected with anelectronic modulator group at the output side via a lock-in amplifieracting as a frequency-selective and phase-selective device, saidmodulator group being coupled to the control input of the modulator forsupplying an appropriate interference signal of the base frequency (f₂)so as to compensate for an unwanted component of the base frequency (f₂)produced from the carrier frequency (f₁) and sideband frequencies (f₁±f₂) by modulation nonlinearities; and a measurement detector having anoutput which is connected with a lock-in detector acting as afrequency-selective and phase-selective device, said measurementdetector being arranged in the beam path after interaction of the laserbeam with the object, said lock-in detector of the modulator group beingadapted or tuned to the base frequency (f₂) which is generated as aresult of photothermal modulation of the index of refraction by mixingthe carrier frequency (f₁) and sideband frequencies (f₁ ±f₂) in theobject.
 27. The arrangement according to claim 26, wherein said lasersource and modulator are combined in an intensity-modulated laser diodewhich is controlled by the modulator.
 28. The arrangement according toclaim 25, wherein another modulator is arranged directly downstream ofsaid modulator and is controlled by a modulation signal of the basefrequency (f₂) which is variable with respect to amplitude, whereinanother modulator group is provided for controlling, which modulatorgroup is connected on the input side with the base (f₂) supplied by themodulator group and which has control input, the lock-in amplifierproviding a corresponding regulating variable via said control input forregulating the amplitude.
 29. In an arrangement for photothermalspectroscopy in which an intensity-modulated laser beam is directed onan object to be examined and in which, as a result of a reaction of theobject to energy applied by the incident laser beam, the objectimpresses an additional modulation of intensity on the laser beam in thepath of the beam after interaction with the object, a measurementdetector is arranged for detecting this additional modulation as anoptical response of the object, the improvement comprising:a lasersource and an optical phase shifter and optical frequency modulator,which are provided for generating the intensity modulated beam, saidelements providing means for effecting the intensity modulation of thelaser beam in such a way that a modulation period of duration T=1/f₂,where f₂ is a base frequency, comprises a time segment duration T₁, inwhich the intensity is modulated with a periodic signal of carrierfrequency f₁, and a time segment duration T₂, in which the intensity isconstant; a semireflecting mirror being arranged in theintensity-modulated laser beam prior to interaction with the object fordecoupling a beam to a regulating detector, and the regulating detectorbeing connected with the electronic modulator group at the output sidevia a lock-in amplifier acting as a frequency-selective andphase-selective device, wherein the modulator group supplies a controlsignal to the phase shifter such that the phase displacement results inan intensity value for the elimination of an interfering component ofthe base frequency (f₂); and a measurement detector having an outputwhich is coupled to a lock-in detector acting as a frequency-selectiveand phase-selective device, said detector beam being arranged in thebeam path after interaction of the laser beam with the object, whereinthe lock-in detector of the modulator group is adapted or tuned to thebase frequency (f₂) which is generated as a result of photothermalmodulation of the index of refraction by mixing the carrier frequency(f₁) and sideband frequencies (f₁ ±f₂) in the object.
 30. Thearrangement according to claim 29, wherein the phase shifter andfrequency modulator are realized in integrated optics and a 3-dBexpander is arranged for splitting and recombining the beam.